That would only be a fair comparison if the US was willing to devote it’s entire military budget to these actions the way Iran can. It would also assume that the US can (and is willing to) spend 1 billion dollars + costs required with overseas operations every time Iran spends 100M on missiles. Iran broke the top 15 for military spending a few years ago so they’re going to have decent capabilities when it comes to being a pain.
It also ignores the cost of dealing with Iranian proxies like Hezbollah and the Houthi, which has Pentagon officials worried as detailed in this article “A $2M missile vs. a $2,000 drone: Pentagon worried over cost of Houthi attacks.”. I’m definitely not cheering for Iran, but I don’t think your total budget vs. total budget comparison is true to the actual economics of a US defense of Israel in the case of sustained attacks. Or even relative cost given that the US has it’s budget spread across many more pursuits than this region.